1. Field of the Invention
The present invention relates to a method for manufacturing a metal powder that is suitable for use in electronics, and more particularly relates to a method for manufacturing a metal powder with a fine, uniform particle size and a high degree of crystallinity which is useful as a conductive powder for use in a conductive paste.
2. Description of the Prior Art
In conductive metal powders used in conductive pastes that are used to form electronic circuits, it is desirable that these powders contain few impurities, that the powders be fine powders with a mean particle size ranging from 0.1 μm or less to approximately 10 μm, that the particle size and particle shape be uniform, and that the particles be monodisperse particles with no aggregation. Furthermore, it is also necessary that the dispersibility of the powder in the paste be good, and that the crystallinity be good so that there is no non-uniform sintering. Especially in cases where such powders are used to form internal conductors or external conductors in multilayer ceramic electronic parts such as multilayer capacitors, multilayer inductors and the like, highly-crystallized or single-crystal metal powders with a spherical shape and low activity, in which the particles are finer sub-micron particles with a uniform particle size and shape, and in which expansion and shrinkage caused by oxidation-reduction tend not to occur during sintering, and the sintering initiation temperature is high, are required in order to prevent structural defects such as delamination, cracking or the like, and in order to enable a reduction in film thickness of the conductor layers.
Specifically, multilayer ceramic electronic parts are generally manufactured by alternately laminating a plurality of unfired ceramic green sheets of dielectric materials, magnetic materials or the like, and internal conductive paste layers whose conductive components are powders of noble metals such as palladium, silver-palladium or the like or base metals such as nickel, copper or the like, and co-firing the thus laminated body at a high temperature. However, in cases where easily oxidizable base metals are used in the internal conductors, various problems arise. For example, in cases where a nickel powder is used as the conductive component of the internal conductive paste, the laminated body is heated in an oxidizing atmosphere up to the point of a binder removal process that is ordinarily performed at a temperature of approximately 300 to 600° C., so that the organic vehicle in the paste and ceramic green sheet is completely removed by combustion. In this case, the nickel powder is slightly oxidized. Afterward, firing is performed in an inert atmosphere or a reducing atmosphere, and a reduction treatment is performed if necessary. However, it is difficult to achieve complete reduction of the nickel powder oxidized in the binder removal process, and this leads to a deterioration in electrical characteristics such as a rise in resistance and the like. Furthermore, expansion and shrinking of the volume of the electrodes occur along with this oxidation-reduction, and since such changes in volume do not coincide with the sintering shrinkage behavior of the ceramic layers, structural defects such as delamination, cracking and the like tend to occur. Furthermore, in a non-oxidizing atmosphere, a nickel powder shows rapid sintering, so that the internal conductors become discontinuous films as a result of over-sintering, leading to the problems of a rise in resistance, disruption of circuits and an increase in the thickness of the conductors, which is contrary to the need for a reduction in film thickness of internal conductor layers made along with an increase in the number of laminated layers in recent years. Such oxidation and over-sintering also present similar problems in the case where external conductors are formed by co-firing using a nickel paste. Accordingly, there is a demand for a highly-crystallized nickel powder which is at least resistant to oxidation at the time of binder removal, and which has a high sintering initiation temperature.
Meanwhile, palladium, which is a noble metal, has the property of undergoing oxidation at relatively low temperatures during firing, and being reduced when heated to even higher temperatures. As a result, structural defects caused by the mismatching of sintering shrinkage behavior between electrode layers and ceramic layers tend to occur. Accordingly, in the case of palladium and palladium alloys as well, resistance to oxidation is desirable and, in terms of resistance to oxidation, highly-crystallized powders with a spherical shape, and especially single-crystal powders, are extremely superior.
Conventionally, a spray pyrolysis method and a vapor phase method have been known as methods for manufacturing such metal powders with a high degree of crystallinity.
The spray pyrolysis method is a method in which a solution or suspension containing one or more compounds is formed into fine liquid droplets, and these liquid droplets are heated preferably at a high temperature near or not lower than the melting point of the metals, so that the metal compounds are pyrolyzed to form a metal or alloy powder. Using this method, a highly-crystallized or single-crystal metal or alloy powder which has a high purity, a high density and a high dispersibility can easily be obtained. In this method, however, large amounts of water or organic solvents such as an alcohol, acetone, ether or the like are used as solvents or dispersing media, so that the energy loss during pyrolysis is large, and the cost is increased. Specifically, in this process, pyrolysis of the metal compound is performed simultaneously with the evaporation of the solvent by heating, or pyrolysis of the metal compound is performed following evaporation of the solvent. In either case, however, a large amount of energy is required in order to evaporate the solvent. Furthermore, since the particle size distribution of the powder becomes broad due to the aggregation and splitting of liquid droplets, it is difficult to set the reaction conditions such as the atomizing velocity, concentration of liquid droplets in the carrier gas, retention time in the reaction vessel and the like, and the productivity is poor. Furthermore, in the case of base metal powders such as nickel, iron, cobalt, copper and the like, if water is used as the solvent, oxidation tends to occur at high temperatures as a result of oxidizing gases generated by the decomposition of the water, so that a powder with a good crystallinity cannot be obtained.
On the other hand, in the case of the vapor phase method, in which a vapor of a metal compound is reduced by a reducing gas at a high temperature, the fine metal powder that is produced tends to aggregate, and control of the particle size is difficult. Furthermore, alloys of metals with different vapor pressures cannot be manufactured with an accurately controlled composition.
Furthermore, there is also a method invented by the present inventors, in which a metal powder with a high crystallinity is manufactured using a solid powder as a raw material by pyrolyzing this raw material at a high temperature in a state in which the raw material is dispersed in a gas phase (see Japanese Patent Publication No. 2002-20809). Specifically, a thermally decomposable metal compound powder is supplied to a reaction vessel using a carrier gas, and a highly-crystallized metal powder is obtained by heating this metal compound powder at a temperature that is higher than the decomposition temperature and not lower than (Tm−200)° C. where Tm (° C.) is the melting point of the metal, in a state in which the metal compound powder is dispersed in the gas phase as a concentration of 10 g/liter or less.
In this case, since the starting raw material is a solid metal compound powder, there is no energy loss caused by solvent evaporation, unlike cases where liquid droplets are used. Furthermore, aggregation and splitting tend not to occur, so that the powder can be dispersed in the gas phase at a relatively high concentration. Accordingly, a spherical monodisperse metal powder which has a high crystallinity and a superior resistance to oxidation can be manufactured at a high efficiency. Furthermore, since there is no generation of oxidizing gases from a solvent, this method is also suitable for the manufacture of easily oxidizable base metal powders which must be synthesized at a low oxygen partial pressure. Furthermore, metal powders with an arbitrary mean particle size and a uniform particle size can be obtained by controlling the particle size and conditions of dispersion of the raw material powder. Moreover, since there is no need to form the raw material into a solution or suspension, the starting raw materials can be selected from various materials, so that numerous types of metal powders can be manufactured. In addition, this method is advantageous in that alloy powders with arbitrary compositions can easily be manufactured by mixing or compositing compounds of two or more types of metals.
On the basis of the method described in the above-mentioned Japanese Patent Publication No. 2002-20809, the present inventors conducted further research in order to find conditions that allow the manufacture of fine, highly-crystallized metal powders with a uniform particle size in a more stable manner and with good reproducibility. This research led to the perfection of the present invention.